Method of producing endless-belt-shaped film

ABSTRACT

A film is made of an insulating matrix resin and an electroconductive material, in which the electroconductive material is dispersed in the insulating matrix resin in such a manner that a surface resistivity of A [Ω] of the film and a volume resistivity of B [Ω·cm] of the film in the direction of a thickness thereof normal to the surface of the film satisfy a relationship of: A&gt;B, and when the film has a thickness of T [cm], the electroconductive material is dispersed in the insulating matrix resin in such a manner that the surface resistivity of A [Ω] of the film and the volume resistivity of B [Ω·cm] of the film satisfy a relationship of; A[Ω]&#39;T [cm]&gt;B [Ω·cm]. This film can be prepared by a centrifugal molding method.

This application is a Division of application Ser. No. 09/391,526 Filedon Sep. 8, 1999, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a film adopted for use in anintermediate image transfer member, which may be used in the form of anendless belt, to which a toner image can be transferred, and which iscapable of reducing the formation of image transfer dust and thedeposition of toner on the background of images in an image formationprocess.

The present invention also relates to an endless-belt-shaped film madeof the above-mentioned film for use in the intermediate image transfermember.

2. Discussion of Background

Conventionally, there is known an image formation apparatus in which alatent electrostatic image is formed on a latent image bearing membersuch as a photoconductor drum, the latent electrostatic image isdeveloped with toner to a visible toner image, the visible toner imageis then electrostatically transferred to an endless-belt-shapedintermediate image transfer belt, and the toner image is furthertransferred from the intermediate image transfer belt to a recordingmaterial such as a transfer sheet.

In this kind of image formation apparatus, an electroconductiveintermediate image transfer belt is widely used as the above-mentionedintermediate image transfer belt.

However, it is difficult to set the surface resistivity of theelectroconductive intermediate image transfer belt at an appropriatevalue. For example, in Japanese Patent 2560727, it is described thatproper images can be formed when the surface resistivity of theelectroconductive image transfer image transfer belt is in a range of10⁷ to 10¹⁵ [Ω/□]. However, there is a case where the deposition oftoner on the background of images occurs and image transfer dust isformed, with a toner being transferred away from its right imagetransfer position even when the surface resistivity of theelectroconductive image transfer belt is set in the above-mentionedrange. It is also known that such image transfer dust occurs frequentlywhen an intermediate image transfer belt with lower surface resistivityis used. Such image transfer dust is considered to be caused by theelectric lines of force, formed between the intermediate image transferbelt and the latent electrostatic image bearing member, being disturbedin the direction of the surface of the intermediate image transfer belt.Furthermore, it is known that the deposition of toner on the backgroundof images occurs frequently when an intermediate image transfer beltwith higher surface resistivity is used. Thus, there is a dilemma inwhich when the surface resistivity of the intermediate image transferbelt is increased in order to reduce the formation of the image transferdust, the deposition of toner on the background of images is increased,while when the surface resistivity of the intermediate image transferbelt is decreased in order to reduce the deposition of toner on thebackground of images, the occurrence of the image transfer dust ispromoted.

In order to get out from such a dilemma, it will be necessary to set thesurface resistivity of the intermediate image transfer belt at a highsurface resistivity and also to provide a charge quenching unit forquenching electric charges of the intermediate image transfer belt.However, the provision of such a charge quenching unit not only makesthe image formation apparatus costly, but also makes the mechanism ofthe image formation apparatus complex. Therefore, the provision of thequenching apparatus is not a preferable step from the above point ofview. Furthermore, a corona charger, which is one of the simplestquenching apparatus, is considered to be usable for the above-mentionedpurpose. However, the corona charge has a shortcoming that it generatesozone and causes an air pollution problem.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide afilm with a single layer structure, which can be adopted for use in anintermediate image transfer member, and may be used in the form of anendless belt, to which a toner image can be transferred, and which iscapable of reducing the formation of image transfer dust and thedeposition of toner on the background of images in an image formationprocess, without the necessity for the provision of any charge quenchingapparatus.

A second object of the present invention is to provide a method ofproducing an endless-belt-shaped film made of the above-mentioned filmfor use in the intermediate image transfer member.

The first object of the present invention can be achieved by a filmwhich comprises an insulating matrix resin and an electroconductivematerial, in which the electroconductive material is dispersed in theinsulating matrix resin in such a manner that a surface resistivity of A[Ω] of the film and a volume resistivity of B [Ω·cm] of the film in thedirection of a thickness thereof normal to the surface of the filmsatisfy a relationship of:

A>B.

The first object of the present invention can also be achieved by a filmcomprising an insulating matrix resin and an electroconductive material,having a thickness of T [cm], the electroconductive material beingdispersed in the insulating matrix resin in such a manner that a surfaceresistivity of A [Ω] of the film and a volume resistivity of B [Ω·cm] ofthe film in the direction of a thickness thereof normal to the surfaceof the film satisfy a relationship of:

A[Ω]×T[cm]>B[Ω·cm].

In any of the above films, the electroconductive material may comprise afirst electroconductive material and a second electroconductivematerial, each of which has a different resistivity.

Furthermore, in any of the above films, the electroconductive materialmay comprise a first electroconductive material and a secondelectroconductive material, each of which has a different particle size.

Furthermore, in any of the above films, the electroconductive materialmay be in the shape of needles.

Each of the needles of the electroconductive material may be in such ashape that the thickness thereof is decreased toward opposite endportions thereof.

As the above-mentioned electroconductive material, an electroconductivematerial comprising carbon can be used.

The first object of the present invention can also be achieved by a filmwith a single layer structure comprising a first region extending alonga surface of the film and a second region extending under the firstregion, the first region comprising an insulting matrix resin and afirst electroconductive material dispersed in the form of particles inthe insulating matrix resin, and the second region comprising theinsulating matrix resin and a second electroconductive materialdispersed in the form of particles in the insulating matrix resin, thefirst electroconductive material having a lower electroconductivity thanthat of the second electroconductive material.

Furthermore, the first object of the present invention can also beachieved by a film with a single layer structure comprising a firstregion extending along a surface of the film and a second regionextending under the first region, the first region comprising aninsulting matrix resin and a first electroconductive material dispersedin the insulating matrix resin, and the second region comprising theinsulating matrix resin and a second electroconductive materialdispersed in the insulating matrix resin, the first electroconductivematerial and the second electroconductive material being of an identicalelectroconductive material, and the first electroconductive materialhaving a larger particle size than that of the second electroconductivematerial.

Furthermore, the first object of the present invention can also beachieved by a film with a single layer structure comprising an insultingmatrix resin and an electroconductive material dispersed in the form ofneedles in the insulating matrix resin, with a longitudinal side of theneedles of the electroconductive material being oriented in thedirection normal to an external surface of the film.

In the above film, it is preferable that a cross section of each of theneedles of the electroconductive material, in the direction normal tothe external surface of the film, be in the shape of a quadrilateralwith unequal diagonal lines, a longer diagonal line being oriented inthe direction normal to the external surface of the film and a shorterdiagonal line being oriented in the direction normal to the longerdiagonal line.

Furthermore, it is preferable that the above quadrilateral have a pairof equal adjacent sides directed to the external surface of the film,and a pair of equal adjacent sides directed to a back side of the film.

Furthermore, it is also preferable that the pair of equal adjacent sidesof the quadrilateral directed to the external surface of the film beshorter than the pair of equal adjacent sides of the quadrilateraldirected to the back side of the film.

The second object of the present invention can be achieved by a methodof producing an endless-belt-shaped film with a single layer structurecomprising an insulting matrix resin and an electroconductive materialdispersed in the form of particles in the insulating matrix resin, withlarger particles of the electroconductive material being positioned onthe side of an external surface of the endless-belt-shaped film, andsmaller particles of the electroconductive material being positioned onthe side of an inner surface of the endless-belt-shaped film, comprisingthe steps of:

dispersing the electroconductive materials with different particles sizein a solution of the matrix resin in a solvent to form a dispersion ofthe electroconductive materials in the solution of the matrix resin, and

subjecting the dispersion to centrifugal molding to localize largerparticles of the electroconductive material on the side of the externalsurface of the endless-belt-shaped film, and to localize smallerparticles of the electroconductive material on the side of the innersurface of the endless-belt-shaped film, with removal of the solventtherefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view of a centrifugal moldingcylinder 1 for forming an endless-belt-shaped film serving as anintermediate image transfer belt on a molding surface thereof bycentrifugal molding.

FIG. 2 is a schematic partial cross-sectional side view of an endlessbelt film 2-1 prepared in Example 1, using the centrifugal moldingcylinder 1.

FIGS. 3A to 3C are schematic-cross sectional views of needles of anelectroconductive material in the endless belt-shaped film, showingvarieties of the cross-section of the needles.

FIG. 3D is a side view of crushed pieces of selenium 3 b, showing theshape thereof, used in Example 2.

FIG. 4 is a schematic partial cross-sectional view of an endless beltfilm 2-2 prepared in Example 2, using the centrifugal molding cylinder1.

FIG. 5 is a schematic partial cross-sectional view of a comparativeendless belt film 2′ prepared in Comparative Example 1, using thecentrifugal molding cylinder 1.

FIG. 6A is a schematic plan view of a face electrode in the arrangementof electrodes in resistivity test specified in JIS K6911 5.13.1.

FIG. 6B is a schematic plan view of a back electrode in the arrangementof electrodes in resistivity test specified in JIS K6911 5.13.1.

FIG. 7A is a diagram in explanation of the connection of electrodes in avolume resistivity test specified in JIS K6911 5.13.1.

FIG. 7B is a diagram in explanation of the connection of electrodes in asurface resistivity test specified in JIS K6911 5.13.1.

FIG. 8 is a diagram in explanation of an insulation resistance measuringapparatus specified in (1.1) in JIS K6911 5.13.1.

FIG. 9 is a diagram in explanation of a measuring method of surfaceresistivity specified in JIS R 3256 3.

FIG. 10A is a plan view diagram in explanation of an example of a methodfor forming electrodes on a test piece for use in a method of measuringsurface resistivity specified in JIS R 3256 3.

FIG. 10B is a front view diagram in explanation of the example of themethod for forming electrodes on a test piece specified in FIG. 10A.

FIG. 11 is a schematic partial cross-sectional view of another exampleof an intermediate image transfer belt of the present invention.

FIGS. 12A to 12F are diagrams in explanation of a method of producing anintermediate image transfer belt of the present invention.

FIG. 13A is a schematic plan view of electrodes for measuring surfaceresistivity.

FIG. 13B is a schematic cross-sectional view of the electrodes shown inFIG. 13A.

FIG. 14 is a schematic diagram of an image formation apparatus providedwith an intermediate image transfer belt of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an image formation process using an image transfer belt to which atoner image is transferred, when the electric resistivity in thedirection of an image transfer surface of the image transfer belt isdifferent from that in the direction of the thickness of the imagetransfer belt, the electric resistivity in the direction of the surfaceof the image transfer belt determines the degree of the disturbance ordeformation of the arrangement of the electric lines of force in thedirection of the surface of the image transfer belt, which electriclines of force are formed between a latent image bearing member whichbears the toner image and the image transfer belt before the transfer ofthe toner image. Furthermore, the electric resistivity in the directionof the thickness of the image transfer belt determines the degree of theeasiness of the flow of electric charges remaining on the image transfersurface of the belt to a non-image transfer surface of the belt, thatis, the easiness of the grounding of the image transfer belt. Therefore,by controlling the electric resistivity in the direction of the imagetransfer surface of the image transfer belt and that in the direction ofthe thickness of the image transfer belt, the deformation of thearrangement of the electric lines of force in the direction of thesurface of the belt, which contribute to the image transfer performance,and the easiness of the grounding to remove the electric chargesremaining on the image transfer surface of the belt can be separatelyadjusted.

In particular, when the electric resistivity in the direction of theimage transfer surface of the image transfer belt is greater than thatin the direction of the thickness of the image transfer belt, theelectric charges on the image transfer surface of the image transferbelt more easily flow in the direction of the thickness of the imagetransfer belt than in the direction of the surface of the image transferbelt, so that the deformation of the arrangement of the electric linesof force in the direction of the image transfer surface of the imagetransfer belt can be reduced, and the grounding of the electric chargeson the image transfer surface of the image transfer belt can befacilitated.

In a method of forming an endless-belt-shaped film, using a centrifugalmolding method, more specifically, in a step of forming theendless-belt-shaped film on a molding surface of a centrifugal moldingcylinder by subjecting a base material for the endless-belt-shaped filmto centrifugal molding, in which base material an electroconductivematerial in the form of needles is dispersed, the needles of theelectroconductive material can be arranged in such a manner that thelongitudinal side of each needle is directed in the direction of thethickness of the belt. In the image transfer belt in which the needlesof the electroconductive material, the resistivity in the direction ofthe surface thereof can be made larger than that in the direction of thethickness thereof.

In particular, when each needle of the electroconductive material ismade in such a shape that one half or a top portion of the needle isheavier than the other portion of the needle, the needles of theelectroconductive material can be easily arranged in the course of thecentrifugal molding in such a manner that the heavier portion of eachneedle is directed to the image transfer surface of the image transferbelt and the lighter portion of each needle is directed to the back sideof the image transfer belt.

More specifically, with reference to FIG. 3A, a cross section of each ofthe needles of the electroconductive material in the direction normal tothe external surface of the film, may be in the shape of a quadrilateralwith unequal diagonal lines, a longer diagonal line a being oriented inthe direction normal to said external surface of the film and a shorterdiagonal line b being oriented in the direction normal to the longerdiagonal line a.

As shown in FIG. 3B, in the above quadrilateral, it is preferable that apair of adjacent sides thereof m, m′ directed to the external surface ofthe film be equal (m=m′), and that another pair of adjacent sides n, n′thereof directed to a back side of the film be also equal (n=n′).

As shown in FIG. 3C, in the above quadrilateral, it is preferable thatthe pair of equal adjacent sides m of the quadrilateral directed to theexternal surface of the film are shorter than the pair of equal adjacentsides n of the quadrilateral directed to a back side of the film (m<n).

FIG. 1 is a schematic cross-sectional view of a centrifugal moldingcylinder 1 for forming an endless-belt-shaped film serving as anintermediate image transfer belt by centrifugal molding, which ishereinafter referred to as an endless-belt-shaped intermediate imagetransfer film 2. The endless-belt-shaped intermediate image transferfilm 2 is formed along a molding surface 1 a of the centrifugal moldingcylinder 1. In FIG. 1, an alternate long and short dash line 1 cindicates an axis of rotation of the centrifugal molding cylinder 1 inthe course of the centrifugal molding. The centrifugal molding cylinder1 is made of a metal such as aluminum, with the molding surface 1 abeing subjected to mirror finish with high precision.

The endless-belt-shaped intermediate image transfer film 2 is formed bycentrifugal molding on the molding surface 1 a, using polyamide acid orpolyamic acid serving as a precursor of polyimide. Polyamide acid hasthe properties of being converted to a polyimide through imide ringclosure.

In the present invention, polyamide acid is used as a starting materialfor forming the endless-belt-shaped intermediate image transfer film ofwhich base material is polymide.

The base material for the endless-belt-shaped intermediate imagetransfer film of the present invention is not limited to theabove-mentioned polyimide, but, for example, the following materials canbe used: polyether sulfone, polycarbonate, polyester, polyarylate,polyphenylene sulfide, polyamide, polysulfone, polyparabanic acid,fluoroplastic, polyamide imide, polyether imide, thermosettingunsaturated polyester, and epoxy thermosetting resin.

Polyamide acid has the properties of performing imide ring closure whendissolved in a particular organic solvent with the application of heatthereto, or in the presence of a catalyst.

A solution of polyamide acid for the formation of polyimide can beobtained by allowing an organic diamine to react with an organictetracarboxylic acid dianhydride in an equimolar ratio in an organicsolvent. In the present invention, a commercially available polyimideprecursor solution (Trademark “TORAYNEECE #3000”, made by TorayIndustries, Inc.) is used as the polyamide acid, and diluted withN,N-dimethyl acetamide (hereinafter referred to as DMAC) appropriately,whereby the polyamide acid solution is prepared.

The organic solvent for dissolving the polyimide precursor which is usedin the preparation of the endless-belt-shaped intermediate imagetransfer film of the present invention, that is, the solvent fordissolving the starting material for forming the endless-belt-shapedintermediate image transfer film, is not limited to CMAC, but anysolvents that are capable of dissolving the starting material can beemployed. Specific examples of such solvents are γ-butyrolactone,dimethyl formamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone,diethylene glycol dimethyl ether, pyridine, dimethyl sulfone,dichloromethane, trichloro-methane, dioxane, and toluene.

The concentration of the starting material in the above solvents is notlimited to a particular concentration, provided that it is preferable toappropriately adjust the viscosity of the solution of the startingmaterial for forming the endless-belt-shaped intermediate image transferfilm in order to obtain an endless-belt-shaped intermediate imagetransfer film with excellent surface properties and with an appropriatethickness with high precision.

The preparation of the endless-belt-shaped intermediate image transferfilm 2, which will be hereinafter referred to as the endless belt film2, will now be explained.

The endless belt film 2 composed of polyamide acid serving as thestarting material, which includes DMAC and an electroconductive materialwhich will be explained later, is formed on the molding surface 1 a ofthe centrifugal molding cylinder 1 by a conventional centrifugal moldingmethod.

In the centrifugal molding method, the starting material may be firstcoated on the molding surface 1 a of the centrifugal molding cylinder 1,and then the centrifugal molding cylinder 1 is rotated to perform thecentrifugal molding. Alternatively, the centrifugal molding cylinder 1is first rotated, and then the starting material is injected thereinto.The latter method is generally called “centrifugal casting”. Forproducing the endless-belt-shaped film of the present invention, eitherthe former method or the latter method can be equally used.

In the present invention, the former method was used. More specifically,the above-mentioned polyamide acid solution was spray coated on themolding surface 1 a of the centrifugal molding cylinder 1 as it wasrotated slowly, and the centrifugal molding cylinder 1 was then rotatedat a high speed of 1000 rpm around the axis of rotation 1 c as shown inFIG. 1, whereby an endless belt film 2 with a thickness of 50 μm wasformed.

In the course of the centrifugal molding, it is necessary to continuethe rotation of the centrifugal molding cylinder 1 until DMAC isevaporated to some extent from the polyamide acid solution placed on themolding surface 1 a in order to prevent the deformation of the endlessbelt film 2 after the centrifugal molding is finished. The rotating timeof the centrifugal molding cylinder 1 can be shortened by applying heatto the polyamide acid solution to promote the evaporation of DMAC as thecentrifugal molding cylinder 1 is rotated. In this case, it ispreferable that the heat application be controlled so as not to induceexcessive imide ring closure in the polyamide acid.

The DMAC still remains in the endless belt film 2 thus produced bycentrifugal molding. The DMAC is removed therefrom in the course of thenext drying and curing process.

The drying and curing process for the endless belt film 2 will now beexplained.

In order to prepare an intermediate image transfer belt from the endlessbelt film 2 prepared by the centrifugal molding, it is necessary to drythe endless belt film 2, and to cure the same with the inducement of theimide ring closure in the polyamide acid contained in the endless beltfilm 2. In other words, it is necessary to convert the endless belt film2 into a cured film made of polyimide which is the base materialthereof. In an example of the intermediate endless image transfer beltof the present invention, the endless belt film 2 was dried at atemperature of 100° C.

There are two methods for inducing the imide ring closure in thepolyamide acid contained in the endless belt film 2. In one method, acatalyst is used for the inducement of the imide ring closure in thepolyamide acid, and in the other method, the polyamide acid is heated.In the formation of the intermediate image transfer belt of the presentinvention, both methods can be used.

In the example of the intermediate endless image transfer belt of thepresent invention, the latter method was employed in which the imidering closure was induced in the polyamide acid by heating the polyamideacid. More specifically, the endless belt film 2 on the molding surface1 a was heated to about 250° C. to induce the imide ring closure in thepolyamide acid contained in the endless belt film 2, whereby the endlessbelt film 2 was cured and changed to a cured film.

As mentioned above, conventionally there is the dilemma that when thesurface resistivity of the intermediate image transfer belt is increasedin order to reduce the occurrence of the image transfer dust, thedeposition of toner on the background of images is increased, while whenthe surface resistivity of the intermediate image transfer belt isdecreased in order to decrease the deposition of toner on the backgroundof images, the occurrence of the image transfer dust is promoted.

The inventors of the present invention have discovered that a solutioncan be given to the above problem by imparting electric anisotropy tothe intermediate image transfer belt. The electric anisotropyspecifically means that the electric resistivity in the direction of theintermediate image transfer belt is greater than the electricresistivity in the direction of the thickness of the intermediate imagetransfer belt.

When a toner image formed on a latent electrostatic image bearing membersuch as a photoconductor is electrostatically transferred to theintermediate image transfer belt, such electric lines of force thatattract the toner image from the latent electrostatic image bearingmember to the intermediate image transfer belt are formed between thelatent electrostatic image bearing member and the intermediate imagetransfer belt. However when the electric resistance in the direction ofthe intermediate image transfer belt is smaller than the electricresistance in the direction of the thickness of the intermediate imagetransfer belt, the arrangement of the electric lines of force in thedirection toward the surface of the intermediate image transfer belt isdisturbed, so that the transfer position of the toner image on theintermediate image transfer belt is deviated from its right transferposition and therefore the formation of the image transfer dust ispromoted.

On the image transfer surface of the intermediate image transfer belt,there remain electric charges due to the effects of the toner depositedon the image transfer surface of the intermediate image transfer beltand also due to the effects of an electric field therearound. Theelectric charges remaining on the image transfer surface of theintermediate image transfer belt increase the attraction of the toner tothe intermediate image transfer belt. Therefore, unless the quantity ofelectric charges of the toner on the intermediate image transfer belt isreduced, it is difficult to remove the toner from the surface of theintermediate image transfer belt, so that the cleaning of theintermediate image transfer belt tends to become insufficient andtherefore the deposition of the toner on the background of images iscaused.

In the present invention, setting the electric resistivity of theintermediate image transfer belt in the direction of the image transfersurface thereof larger than that of the intermediate image transfer beltin the direction of the thickness thereof facilitates the flow ofelectric charges in the direction of the thickness of the intermediateimage transfer belt from the image transfer surface of the intermediateimage transfer belt, whereby the deformation of the arrangement of theabove-mentioned electric lines of force can be reduced, and the electriccharges on the image transfer surface of the intermediate image transferbelt can be easily grounded through the intermediate image transferbelt, and accordingly the formation of the image transfer dust and thedeposition of toner on the background of the images can be significantlyreduced. Such electric charges are generally grounded, for instance,through a rotating roller which is rotated in sliding contact with anon-image-transfer surface of the intermediate image transfer belt.

Preparation of intermediate image transfer belts with theabove-mentioned electric resistance anisotropy will now be explainedwith reference to the following examples:

EXAMPLE 1

Commercially available fine carbon fibers cut with a length of 5 μm orless serving as an electroconductive material, and polyamide acid weremixed with a ratio of 0.15:1. This mixture was dispersed in DMAC anddiluted with DMAC so as to prepare a polyamide acid solution with asolid component ratio of 30%.

The thus prepared polyamide acid solution was subjected to centrifugalmolding on the molding surface 1 a of the centrifugal molding cylinder1. In the course of the centrifugal molding, the cut carbon fibers inthe polyamide acid solution were moved toward the molding surface 1 a,with the longitudinal direction thereof being directed so as to be inparallel with the direction of the centrifugal force in the peripheralradius direction by the centrifugal force of the rotating moldingsurface 1 a of the centrifugal molding cylinder 1.

FIG. 2 is a schematic partial cross-sectional view of an endless beltfilm 2-1 thus prepared in Example 1, which was formed in the centrifugalmolding cylinder 1.

As shown in FIG. 2, in the course of the movement of the carbon fibers,carbon fibers 3 a in the endless belt film 2-1 were oriented with thelongitudinal direction thereof being directed in the direction of thethickness of the endless belt film 2-1.

The thus molded endless belt film 2-1 was dried and cured, whereby anintermediate image transfer belt was prepared.

The thus prepared intermediate image transfer belt was incorporated in acommercially available full-color copying machine (Trademark “PRETER”,made by Ricoh Company, Ltd.) and color copies were made. The result wasthat high quality color images free from image transfer dust and thedeposition of toner on the background of images were obtained. In thisfull-color copying machine, each toner color image formed on aphotoconductor drum was transferred to the intermediate image transferbelt, using electrostatic force, in a superimposed manner to prepare thefull-color image.

The electric resistance in the direction of an external surface of theintermediate image transfer belt prepared in Example 1 measured about10⁹ Ω, while the electric resistance in the direction of a thickness ofthe intermediate image transfer belt measured 5×10⁸ Ω.

A volume resistivity [Ω·cm] of the intermediate image transfer belt inany examples including this example can be determined by multiplying theelectric resistance [Ω] in the direction of the thickness of theintermediate image transfer belt by the thickness of the intermediateimage transfer belt, 50 μm.

EXAMPLE 2

In this example, as the electroconductive material, a material preparedby crushing spherulites of selenium to a grain size of 1 μm or less wasemployed.

Spherulite is a kind of crystalline structure observed in many kinds ofpolymeric materials. Spherulites are radial aggregates of sphericalpolycrystals composed of crystals of ortho-rhombic system or crystals ofmonoclinic system. Each crystal in the spherulites grows in a radialdirection, so that each crystal is in such a shape that its widthincreases outwards from a central point, or in the shape of a wedge.Therefore, each crystal has its center of gravity shifted toward anouter side in the longitudinal direction thereof. In other words, whenthe spherulites are crushed and separated in the longitudinal directionof each crystal, wedge-shaped pieces of the electroconductive materialare formed, having a heavier end portion and a lighter end portion.

As a matter of fact, it was confirmed that crushed pieces of selenium 3b, which were obtained by crushing crystallized selenium to a grain sizeof 1 μm or less, were in a different shape from that of the cut carbonfibers employed in Example 1, each of which was in the shape of awhisker. More specifically, FIG. 3 shows the shape of each of thecrushed pieces of selenium 3 b, which is a side view of the crushedpieces of selenium 3 b. As shown in FIG. 3, it was confirmed that eachof the opposite ends of the crushed piece of selenium 3 b has the shapeof a cone by SEM inspection. Each of the spherulites of selenium beforethe spherulites were crushed had a grain size of about 50 μm.

The crushed pieces of selenium 3 b and polyamide acid were mixed with aratio of 0.20:1. This mixture was dispersed in DMAC and diluted withDMAC so as to prepare a polyamide acid solution with a solid componentratio of 30%.

The thus prepared polyamide acid solution was subjected to centrifugalmolding on the molding surface 1 a of the centrifugal molding cylinder1. In the course of the centrifugal molding, the crushed pieces ofselenium 3 b in the polyamide acid solution were moved toward themolding surface 1 a, with the heavier cone-shaped end portion thereof(the cone-shaped end portion of the piece of selenium 3 b on the leftside in FIG. 3) being directed to the molding surface 1 a of thecentrifugal molding cylinder 1, by the centrifugal force of the rotatingmolding surface 1 a of the centrifugal molding cylinder.

FIG. 4 is a schematic partial cross-sectional view of an endless beltfilm 2-2 thus prepared in Example 2, which was formed in the centrifugalmolding cylinder 1. As shown in FIG. 4, in the course of the movement ofthe crushed pieces of selenium 3 b, the crushed pieces of selenium 3 bin the endless belt film 2 were oriented with the longitudinal directionthereof being directed in the direction of the thickness of the endlessbelt film 2.

The movement of such an electroconductive material and the orientationthereof with the longitudinal direction thereof being directed in thedirection of the thickness of the endless belt film in the course of thecentrifugal molding is not limited to the grains of theelectroconductive material having such a cone-shaped end portion on theopposite sides thereof as in the above-mentioned crushed pieces ofselenium 3 b, but the above-mentioned movement can also occur in anygrains of the electroconductive material if each of the grains has aheavier end portion and a lighter end portion in a longitudinaldirection thereof.

In the case of the endless belt film 2-2 shown in FIG. 4, the grains ofthe electroconductive material were more easily arranged so as to bedirected in the direction of the thickness of the endless belt film 2-2than in the case of the endless belt film 2-1 shown in FIG. 2 in Example1.

The thus molded endless belt film 2-2 was dried and cured, whereby anintermediate image transfer belt was prepared.

The thus prepared intermediate image transfer belt was incorporated inthe same full-color copying machine as employed in Example 1 and colorcopies were made. The result was that high quality color images freefrom image transfer dust and the deposition of toner on the backgroundof images were obtained.

The electric resistance in the direction of the external surface of theintermediate image transfer belt prepared in Example 2 measured about10⁹ Ω, while the electric resistance in the direction of the thicknessof the intermediate image transfer belt measured 5×10⁹ Ω.

EXAMPLE 3

In this example, as the electroconductive material, a crushed carbonizedmaterial 3 c was employed, which was prepared by carbonizing spherulitesof an organic polymer in an atmosphere of an inert gas, and crushing thecarbonized spherulites to a grain size of 1 μm or less. An SEMinspection of the thus prepared crushed carbonized material 3 cindicated that pieces of the crushed carbonized material 3 c had thesame shape as that of the pieces of the crushed selenium 3 b employed inExample 2.

The crushed carbonized material 3 c and polyamide acid were mixed with aratio of 0.15:1. This mixture was dispersed in DMAC and diluted withDMAC so as to prepare a polyamide acid solution with a solid componentratio of 30%.

The thus prepared polyamide acid solution was subjected to centrifugalmolding on the molding surface 1 a of the centrifugal molding cylinder1. In the course of the centrifugal molding, the grains of the crushedcarbonized material 3 c in the polyamide acid solution were moved towardthe molding surface 1 a, with the heavier cone-shaped end portionthereof being directed to the molding surface 1 a of the centrifugalmolding cylinder 1 in exactly the same manner as in the pieces of thecrushed selenium 3 b employed in Example 2, by the centrifugal force ofthe rotating molding surface 1 a of the centrifugal molding cylinder.

In the above, the grains of the crushed carbonized material serving asthe electroconductive material were more easily arranged so as to bedirected in the direction of the thickness of the endless belt film thanin the case of the endless belt film shown in FIG. 2 in Example 1.

The state of the grains of the crushed carbonized material 3 c in theendless belt film was the same as that of the crushed pieces of selenium3 b in the endless belt film 2-2 as shown in FIG. 4, so that a figureshowing the state of the grains of the crushed carbonized material 3 cin the endless belt film is omitted.

The thus molded endless belt film was dried and cured, whereby anintermediate image transfer belt was prepared.

The thus prepared intermediate image transfer belt was incorporated inthe same full-color copying machine as employed in Example 1 and colorcopies were made. The result was that high quality color images freefrom image transfer dust and the deposition of toner on the backgroundof images were obtained.

The electric resistance in the direction of the external surface of theintermediate image transfer belt prepared in Example 2 measured about10⁹ Ω, while the electric resistance in the direction of the thicknessof the intermediate image transfer belt measured 5×10⁹ Ω.

Conventional intermediate image transfer belts without anisotropy withrespect to electric resistivity will now be explained as comparativeexamples.

Comparative Example 1

A commercially available spherical carbon black serving as anelectroconductive material, and polyamide acid were mixed with a ratioof 0.15:1. This mixture was dispersed in DMAC and diluted with DMAC soas to prepare a polyamide acid solution with a solid component ratio of30%.

The thus prepared polyamide acid solution was subjected to centrifugalmolding on the molding surface 1 a of the centrifugal molding cylinder1.

FIG. 5 is a schematic partial cross-sectional view of a comparativeendless belt film 2′ thus prepared in Comparative Example 1, which wasformed in the centrifugal molding cylinder 1.

As shown in FIG. 5, spherical carbon black particles 3 d were regularlydispersed in the comparative endless belt film 2′.

The thus molded endless belt film 2′ was dried and cured, whereby acomparative intermediate image transfer belt No. 1 was prepared.

The thus prepared comparative intermediate image transfer belt No. 1 wasincorporated in the same commercially available full-color copyingmachine as employed in Example 1, and color copies were made. The resultwas that there was formed no image transfer dust, but the deposition oftoner on the background of images occurred in the obtained images.

The electric resistance in the direction of the external surface of theintermediate image transfer belt prepared in Comparative Example 1measured about 10⁹ Ω, while the electric resistance in the direction ofthe thickness of the intermediate image transfer belt measured 5×10¹³ Ω.

Comparative Example 2

The procedure of preparing the comparative intermediate image transferbelt No. 1 in Comparative Example 1 was repeated except that thecommercially available spherical carbon black serving as theelectroconductive material, and polyamide acid were mixed with a ratioof 0.20:1, whereby a comparative endless belt film was prepared inComparative Example 2. The spherical carbon black particles 3 d weredispersed almost in the same manner in the comparative endless belt film2 as in the comparative endless belt film 2′ in Comparative Example 1,so that a figure showing the comparative endless belt film is omittedhere. However, the dispersing density of the spherical carbon blackparticles 3 d in the comparative endless belt film was greater than thatin the comparative endless belt film 2′ in Comparative Example 1.

The thus molded endless belt film was dried and cured, whereby acomparative intermediate image transfer belt No. 2 was prepared.

The thus prepared comparative intermediate image transfer belt No. 2 wasincorporated in the same commercially available full-color copyingmachine as employed in Example 1, and color copies were made. The resultwas that the deposition of toner on the background of images did notoccur in the obtained images, but there was formed image transfer dust.

The electric resistance in the direction of the external surface of theintermediate image transfer belt prepared in Comparative Example 2measured about 10⁸ Ω, while the electric resistance in the direction ofthe thickness of the intermediate image transfer belt measured 5×10¹¹ Ω.

The following TABLE 1 shows the characteristics of the electricresistance and the quality of the formed image in each of theabove-mentioned examples and comparative examples:

TABLE 1 Electric State of formed images Electric resistance Depositionresistance (Ω) in the of toner (Ω) in the direction Image on thedirection of transfer background of surface thickness dust of images Ex.1 10⁹ 5 × 10⁸ None None Ex. 2 10⁹ 5 × 10⁸ None None Ex. 3 10⁹ 5 × 10⁸None None Comp 10⁹  5 × 10¹³ None Found Ex. 1 Comp. 10⁹  5 × 10¹¹ FoundNone Ex. 2

The results shown in TABLE 1 indicates that when the conditions that theelectric resistance in the direction of the surface of the intermediateimage transfer belt is greater than that in the direction of thethickness of the intermediate image transfer belt are satisfied, boththe formation of the image transfer dust and the deposition of the toneron the background of the images can be reduced.

The measurement of the electric resistance of the intermediate imagetransfer belt will now be explained.

The conversion from surface resistance to volume resistivity and viceversa will now be explained.

In the case of an isotropic electroconductive thin film with a thicknessT [cm], a surface resistance ρs [Ω], and a volume resistivity ρv [Ω·cm],for instance, with reference to a technical article entitled“Resistivity Correction Coefficient” (published by Mitsubishi Yuka Co.,Ltd.), it is known that the following relationship holds between thethickness T [cm], the surface resistance ρs [Ω], and the volumeresistivity ρv [Ω·cm]: ρs=ρv/T

From the above formula, the surface resistance can be converted into thevolume resistivity and vice versa.

In the present invention, the surface resistivity and the volumeresistivity were measured in accordance with the respective proceduresdefined in the Japanese Industrial Standards (JIS K 6911 5.13.1).

More specifically, the surface resistivity was measured by connecting anouter ring face electrode and an inner circular disc face electrode on atest piece, using an insulating resistance measuring apparatus specifiedin JIS K 6911 5.12, under the conditions as defined in JIS K 6911 5.13.

The volume resistivity was measured by connecting a face electrode and aback electrode of the test piece, using the above-mentioned insulatingresistance measuring apparatus, under the conditions as specified in JISK 6911 5.13.

Other methods as specified in the Japanese Industrial Standards (JIS3256) can also be used for measuring the surface resistivity and thevolume resistivity.

The Japanese Industrial Standards (JIS K 6911 5.13.1) are as follows:

FIG. 6A is a schematic plan view of a face electrode, and FIG. 6B is aschematic plan view of a back electrode. In these figures, the unit ofthe dimensions is mm.

FIGS. 7A and 7B respectively show an arrangement of the, electrodes in avolume resistivity test and an arrangement of the electrodes in asurface resistivity test of the Japanese Industrial Standards (JIS K6911 5.13.1).

(1) Apparatus

(1.1) A conductive rubber cut into the shape shown in FIG. 6A and FIG.6B by hatching, or a moisture permeable conductive paint.

(1.2) A power source, insulation resistance measuring instrument andswitches in (1.1.2) through (1.1.4) of 5.12.1 of the Japanese IndustrialStandards (JIS K6911 5.12.1).

(1.3) Micrometer Calliper

The micrometer calliper for external measurement specified in JIS B 7502or one with an accuracy at least equivalent.

(1.4) Vernier Calliper

The vernier calliper for external measurement specified in JIS B 7507 orone with an accuracy at least equivalent.

(2) Test Piece

Use a test piece molded into a disc about 100 mm in diameter and 2 mm inthickness.

(3) Preconditioning

Precondition the test pieces under C-90⁺⁴ ⁻² h/20±2° C./(65±5)% RH.

(4) Procedure

Measure the thickness of the test piece finished with conditioning, withthe micrometer calliper for external measurement accurate to 0.01 mm,and press the conductive rubber upon the test piece in the positionsshown in FIG. 6A and FIG. 6B to be the electrodes.

Alternatively, the electrodes may be provided by painting on the testpiece with the moisture permeable conductive paint shown in FIG. 6A andFIG. 6B. In this case, treat the test piece after painting theelectrodes paying attention so that the moisture permeable conductivepaint does not peal off from the test piece during operation.

Measure the outside diameter of inner circle of face electrode and theinside diameter of ring electrode with the vernier calliper to thenearest 0.02 mm. Make connections shown in FIG. 7A for measuring volumeresistivity and shown in FIG. 7B for measuring surface resistivity.Connect this assembly in the position of test piece in the same circuitas that given in 5.12, charge it for 1 min and measure the volumeresistivity and surface resistivity.

In the above, carry out the tests under the condition in 5.1 (1) [20±2°C. temperature, (65±5) % relative humidity].

(5) Calculation

Calculate volume resistivity and surface resistivity by the followingequations:

ρv=(πd ²/4T)×Rv

ρs={π(D+d)/(D−d)}×Rs

where

ρv: volume resistivity (MΩcm)

ρs: surface resistivity (MΩ)

d: outside diameter of inner circle of face electrode (cm)

t: thickness of test piece (cm)

Rv: volume resistance (MΩ)

D: inside diameter of ring electrode on face (cm)

Rs: surface resistance (MΩ)

π: ratio of circle's circumference to its diameter=3.14

The following is a portion of 5.12.1 of JIS K 6911. FIG. 8 is a diagramof Insulation resistance measuring apparatus specified in (1.1) of JIS K6911 5.12.1.

(1.1) Insulation Resistance Measuring Apparatus

An apparatus consisting of electrodes, power source, galvanometer,universal shunt, switches, etc. as exemplified in FIG. 8.

(1.1.1) Electrodes

The brass taper pins of Class B specified in JIS B 1352, with 5 mmdiameter and free from scars on the surfaces.

(1.1.2) Power Source

A dry or storage battery at 500 V d.c. voltage. A power source fromrectified a.c. may be used, provided that it is certain that it keeps acertain d.c. voltage.

(1.1.3) Insulation Resistance Measuring Instrument

(1,1.3.1) Measuring Insulation Resistance of Not Less Than 1 MΩ But LessThan 10⁶ MΩ (comparison method)

The standard resistance shall be of 1 MΩ manganese or one with anaccuracy at least equivalent, and the universal shunt shall be accurateenough for adjusting the deflection and measuring range of thegalvanometer.

A galvanometer can measure a resistance of less than 10⁶ MΩ with ±10%accuracy, if it is highly sensitive with stable zero point as to show 1mm deflection at 1 m distance by a current of 10⁻¹⁰ A.

(1.1.3.2) Measuring Insulation Resistance of Not More Than 5 MΩ

Use the insulation resistance tester specified in JIS C 1302.

(1.1.3.3) Measuring Insulation Resistance of Not Less Than 1 MΩ But LessThan 10⁸ MΩ

Use an apparatus having a d.c. amplifier calibrated to ±10% accuracy.

(1.1.4) Switches

Switches properly insulated and protected.

The following is JIS R 3256 3, in which another method of measuringelectric resistivity is specified.

FIG. 9 is a diagram showing the method of measuring surface resistivity.

3 Measuring Method at Ordinary Temperature

3.1 Principle of Measurement

Apply voltage to the surface of test piece using the measuring circuitcomposing of an electrode—test piece system X, d.c. power source E, d.c.voltmeter V and ammeter A as shown in FIG. 9 and calculate the surfaceresistivity by the formula specified in 3.5 from the value of surfaceelectrical resistance obtained by dividing this applied voltage by thecurrent flowing on the surface of test piece.

As a simplified measuring device for surface resistivity in which theequipment shown in FIG. 9 is intergrated into one set, high isolationresistance meters are commercially available.

3.2 Measuring Condition and Applied Voltage

The measurement shall be carried out after allowing the test piece tostand at least for 16 hours in the room conditioned at (20±2)° C. intemperature and (50±5) % in relative humidity.

The applied voltage to the test piece shall be 1,000 V or lower and thestandard voltage shall be 500 V.

The duration of voltage application shall be one minute normally andvaried according to quality of the glass substrates.

3.3 Preparing Method of Test Piece

The test piece shall be prepared as follows:

(a) Shape and Dimension

The test piece shall be a rectangle one side of which is 50 mm or moreor a disk whose diameter is 50 mm or more.

(b) Surface Condition of Test Piece

The test piece of mirror finish or similar surface condition shall beused.

(c) Washing and Drying

The test piece shall be washed at first by rubbing using neutraldetergent followed by rinsing using waterworks and then by ultrasoniccleaning within such solvent as extra pure water, acetone, ethanol, etc.

Drying may be made by using an oven or by natural drying.

(d) Methods for Forming Electrode on Test Piece

The formation of electrode on the test piece shall be done byvaporization or sputtering of conductive material, or the like. Gold,platinum, etc. are used as the conductive material but gold ispreferable for this measuring method.

In this measuring method high insulation resistance is measured,therefore, it is necessary to form a guard electrode in order to removestray current in the electrode—test piece system.

(e) Dimension of Electrodes

The electrodes arranged to form concentric circles as shown in FIG. 10Aand FIG. 10B shall be used. In this arrangement, the size of gap may beadjusted by altering the dimension of the main electrode within a rangeof approximately 26 mm to 36 mm, taking sensitivity of the measuringdevice into account.

3.4 Measuring Procedure

(a) Form the electrodes on the test piece in accordance with the methodshown in FIG. 10A and FIG. 10B and measure the diameter D₁ of the mainelectrode and the internal diameter D₂ of the counter electrode at anaccuracy of 0.05 mm using the vernier callipers specified in JIS B 7507or the measuring instruments at least equivalent thereto in accuracy.

(b) After drying the test piece for at least two hours at approximately120° C., cool it in a desiccator.

(c) Measure the surface resistivity by the measuring method of surfaceresistivity shown in FIG. 9 under the measuring conditions and appliedvoltage given in 3.2. The measuring device should be maintained in anultra high resistance measuring box shielded.

3.5 Calculation and Number of Measurements

The surface resistivity of the glass substrate shall be calculated bythe following formula;

ρs[Ω]=(Dmπ/g)Rs

where,

ρs: surface resistivity (Ω)

Rs: surface electrical resistance (Ω)

Dm: average diameter (D₁+D₂)/2 (mm)

D₁: diameter of main electrode (mm)

D₂: internal diameter of counter electrode (mm)

g: gap (D₂−D₁)/2 (mm)

The measurements shall be carried out twice and the average value of themeasurements shall be taken as the value of surface resistivity.

FIG. 11 is a schematic partial cross-sectional view of an intermediateimage transfer belt 11, which is another example of the intermediateimage transfer belt of the present invention.

As shown in FIG. 11, the intermediate image transfer belt 11 comprises asubstrate 12 and a plurality of first carbon particles 13 and aplurality of second carbon particles 14. The first carbon particles 13have a lower electroconductivity than that of the second carbonparticles 14, and have a larger particle size than that of the secondcarbon particles 14.

Furthermore, as shown in FIG. 11, the presence of the first carbonparticles 13 is localized on the side of the external surface of theintermediate image transfer belt 11, so that the external surface of theintermediate image transfer belt 11 is caused to have high resistivity,which is advantageous to obtain high quality image. Furthermore, theintermediate image transfer belt 11 has a single-layer structure andtherefore does not have such a shortcoming that a conventional two-layerintermediate image transfer belt has, that is, a shortcoming that asurface layer is peeled off a substrate for the surface layer.

For instance, a polymer for use in the intermediate image transfer belt11 which is used as an intermediate image transfer medium for use in acolor copying machine is required to have fire retardance, highstrength, and electric stability. As such a polymer, for instance,fluoroplastic and polyimide resin are employed.

In particular, polyimide is a promising material from its strength andtriboelectric chargeability. A centrifugal molding method can be givenas a method of forming an endless-belt-shaped intermediate imagetransfer belt, using polyimide resin.

The polyimide resin serving as a basic material for the intermediateimage transfer belt can be synthesized from its precursor, polyamideacid. As mentioned above, polyamide acid has the properties of beingchanged to polyimide with the occurrence of imide ring closure with theapplication of heat or in the presence of a catalyst, and also of beingsoluble in a particular solvent. A dispersion of carbon particles withdifferent particle sizes in the solution of polyamide acid in aparticular solvent is hereinafter referred to as a mixed polyamide acidsolution.

Carbon can be classified into acetylene black, oil furnace black,thermal black, and channel black. Acetylene black can be obtained bysubjecting acetylene to pyrolysis in a preheated furnace. Oil furnaceblack can be obtained by injecting petroleum into a furnace, subjectingthe petroleum to incomplete combustion with the adjustment of the amountof air to be supplied to form carbon, cooling the thus formed carbon,and collecting the carbon, using a cyclone. Thermal black can beobtained by subjecting a natural gas to alternate heat accumulation andpyrolysis at 200° C. to 1700° C. in a heat accumulation furnace. Channelblack can be obtained by blowing a fire of a natural gas against anarrow iron plate so as to deposit carbon on the iron plate.

It is unnecessary to select a particular carbon from the above carbonsfor use in the intermediate image transfer belt of the presentinvention. However, it should be avoided that carbon capable ofimparting high electroconductivity even in a small amount to theintermediate image transfer belt, such as acetylene black (made by DenkiKagaku Kogyo Kabushiki Kaisha) and Ketjen Black EC (made by LionCorporation), is localized near the surface of the intermediate imagetransfer belt when it is desired to set the surface of the intermediateimage transfer belt at high surface resistivity.

The carbon can be dispersed in the organic solvent, using dispersionmeans, such as an ultrasonic dispersion means, a ball mill or a sandmill. Generally, the carbon is not directly dispersed in the polyamideacid solution, but is first dispersed in N-methylpyrrolidone(hereinafter referred to as NMP) to prepare a dispersion of the carbon,and the dispersion of the carbon is then mixed with a polyamide acidsolution.

For example, in the case where the carbon is dispersed in a solvent,using a sand mill, the particle size of the carbon dispersed in thesolvent changes, depending upon the dispersion time, the amount of themedium for the dispersion, the number of revolutions of a disk in thesand mill, and the viscosity of the carbon-dispersed liquid. Therefore,it is necessary to determine the dispersing conditions in advance inaccordance with the desired particle size of carbon particles byconducting preliminary experiments.

Thus, a dispersion in which carbon particles with differentelectroconductivities and with different particle sizes are dispersed isprepared. In this case, the dispersion is conducted in such a mannerthat carbon particles with a lower electroconductivity are adjusted tohave a larger particle size.

A predetermined amount of the mixed polyamide acid solution, which is amixture of the solution of polyamide acid and the dispersion of carbonparticles, is poured into a centrifugal molding cylinder while it isrotated slowly, and the rotation speed is gradually increased thereafterto reach a predetermined rotation speed. The rotation is continued atthe predetermined rotation speed for a predetermined period of time.

By this rotation, the mixed polyamide acid solution is subjected to castmolding inside the centrifugal molding cylinder and at the same time,the carbon particles begin to be separated. Carbon particles with largerparticle sizes are selectively moved toward the external surface of theintermediate image transfer belt and calized near the external surface,changing the distribution state of the carbon particles in the mixedpolyamide acid solution. However, if this rotation is continued withhigher rotation speed, or for an extended period of time, even carbonparticles with smaller particle sizes also tend to move toward theexternal surface. In the present invention, it must be avoided thatcarbon particles with different particle sizes get together near theexternal surface of the intermediate image transfer belt. Therefore, thenumber of revolutions and the rotating time of the centrifugal moldingcylinder are appropriately set so as to avoid the localization of thecarbon particles with different particle sizes near the external surfaceof the intermediate image transfer belt.

In the course of the rotation of the centrifugal molding cylinder, theorganic solvent is caused to evaporate from the mixed polyamide acidsolution, and the solidification of the polyamide acid proceeds, wherebya cylindrical film is formed. This evaporation is preferable because itis done in a heated atmosphere, and therefore is carried out moreeffectively and more quickly than the evaporation in an atmosphere atnormal temperature. It is preferable that the inside of the centrifugalmolding cylinder, which is a casting mold, be mirror finished withhighest precision. The size of the centrifugal molding cylinder shouldaccord with the size of the cylindrical film to be made.

In order to make the thus obtained polyamide acid film satisfy variouscharacteristics such as heat resistance, resistance to chemicals, andmechanical characteristics required, it is necessary to further heat thepolyamide acid film to perform the imide ring closure. The imide ringclosure is conducted by the application of heat to the polyamide acidfilm, with complete elimination of any and all solvents remaining in thepolyamide acid film by evaporating the solvents. In practice, the imideclosure may be carried out by heating the polyamide acid film to apredetermined temperature as it is rotated for a predetermined period oftime, right after the casting of the polyamide acid film. Alternatively,the polyamide acid film may be released from the centrifugal moldingcylinder, and then set in a different cylindrical mold so as to coverthe mold with polyamide acid film, and then heated by heating meansusing, for example, hot air, whereby a polyimide film can be obtained.

The thus obtained polyamide film can be used as it is or after it isworked appropriately, as a functional member for various applications.

When this polyimide film is used as the intermediate image transfer beltfor use in a full-color copying machine, the film is cut in anappropriate size, and if necessary, a skewing stop member is attached tothe opposite end portions thereof.

The thus obtained intermediate image transfer belt includes carbonparticles with different electroconductivities, with the carbonparticles with a lower electroconductivity being localized on the sideof the external surface of the intermediate image transfer belt, so thatthe surface of the intermediate image transfer belt has a higher surfaceresistivity. The thus obtained intermediate image transfer belt 11 doesnot have a conventional two-layer structure composed of a substrate anda surface layer, so that the intermediate image transfer belt does nothave a shortcoming that the surface layer is peeled off the substrate.

FIGS. 12A to 12F are diagrams in explanation of a method of producingthe intermediate image transfer belt of the present invention. In orderto produce the intermediate image transfer belt, to begin with, as shownin FIG. 12A, the following are prepared: a starting material 22, whichis a 20 wt. % solution of polyamide acid in NMP, a first dispersion 23,which is a dispersion of acetylene black (Trademark “Denka Back”, madeby Denki Kagaku Kogyo Kabushiki Kaisha) with a particle size of 0.07 μmin NMP, prepared using a sand mill, and a second dispersion 24, which isa dispersion of furnace back (Trademark “Asahi #60”, made by AsahiCarbon Co., Ltd.) with a particle size of 0.2 μm in NMP, prepared usinga sand mill.

As shown in FIG. 12B, the starting material 22, the first dispersion 23and the second dispersion 24 are mixed to prepare a polyamide acid mixedsolution 21. The composition of the polyamide acid mixed solution 21 issuch that the solid component content of “Denk Black” is 6 phr and thesolid component of “Asahi #60” is 4 phr with respect to the solidcomponent of the polyimide.

As shown in FIG. 12C, the polyamide acid mixed solution 21 is injectedinto a centrifugal molding cylinder 20 with an inner diameter of 100 mmand a length of 250 mm through an injection tube 19. The centrifugalmolding cylinder 20 is rotated at 10 rpm in the direction of the arrow25 when the polyamide acid mixed solution 21 is injected thereinto. Thisrotating speed is maintained until the injection of the polyamide acidmolding solution 21 is finished.

As shown in FIG. 12D, when the injection of the polyamide acid moldingsolution 21 has been finished, the number of revolutions of thecentrifugal molding cylinder 20 is increased up to 400 rpm in thedirection of the arrow 26, and thereafter the centrifugal moldingcylinder 20 is gradually heated to 100° C., using a sheet-shaped heater24, and the temperature is maintained. Thus, the solvent is caused toevaporate from a polyamide acid solution layer 21 a formed on an innerperipheral surface of the centrifugal molding cylinder 20. As a matterof course, the centrifugal molding cylinder 20 may be heated by heatingmeans other than the above-mentioned sheet-shaped heater 24, such as aheating furnace.

When the solvent has been sufficiently caused to evaporate from thepolyamide acid solution layer 21 a, a polyamide acid belt 21 b isformed. The polyamide acid belt 21 b is removed from the centrifugalmolding cylinder 20, and mounted on a mold 27 for changing the polyamideacid belt 21 b to a polyimide belt as shown in FIG. 12E.

As shown in FIG. 12F, the mold 27, with the polyamide acid belt 21 bbeing mounted thereon, is placed in a furnace 28 in which thetemperature is maintained at 300° C., and the polyamide acid belt 12 bis heated for 20 minutes, whereby an aromatic polyimide belt isobtained.

The surface resistivity at a face side and that of a back side of thearomatic polyimide belt are measured, using electrodes shown in FIG. 13Aand FIG. 13B, in accordance with the procedure described in the JapaneseIndustrial Standards (JIS-K 6911).

In this measurement, a ring electrode 31 and a cylindrical electrode 32are placed concentrically on a test piece 34 as shown in FIG. 13A andFIG. 13B. In the measurement, a grounding electrode 33 is disposed on aback side of the test piece 33. When a surface resistivity between thering electrode 31 and the cylindrical electrode 32 is Rs, the surfaceresistivity ρs is:

ρs=18.85Rs[Ω]

The surface resistivity at the face side of the aromatic polyimide belt,ρsA and that at the back side of the aromatic polyimide belt, ρsB,measured by the above-mentioned method, are respectively as follows:

ρsA=6×10¹⁴ Ω and ρsB=3×10⁷ Ω

In the method of producing the intermediate image transfer belt of thepresent invention, carbon particles with different particle sizes aremixed with the base material for forming the substrate of theintermediate image transfer belt, and the mixture is then subjected tothe centrifugal molding, whereby carbon particles with larger particlesizes are localized to a surface side of the intermediate image transferbelt, thus regions with different electroconductivities are formed inthe direction of the thickness of the intermediate image transfer belt.

Furthermore, when as the carbon particles with larger particle sizes,carbon particles with a lower electroconductivity are selectively used,the carbon particles with a lower electroconductivity are localized onan outer side in the course of the centrifugal molding, so that asurface layer with higher resistivity can be formed. Thus, according tothe present invention, it is unnecessary to provide a separate surfacelayer on a substrate as in the conventional method for producing theintermediate image transfer belt, so that the number of production stepscan be reduced.

The thus produced intermediate image transfer belt can be used, forexample, in an image formation apparatus as shown in FIG. 14.

The image formation apparatus as shown in FIG. 14 is provided with aphotoconductor drum 41 serving as a chargeable image bearing member, acharger 42 for charging the photoconductor drum 41, an exposure unit 43for having the charged photoconductor drum 41 exposed to a light imageto form a latent electrostatic image on the photoconductor drum 41, adevelopment apparatus composed development units 44B for development ofblack, 44C for development of cyan, 44M for development of magenta, and44Y for development of yellow, for developing the latent electrostaticimage to toner images, an intermediate image transfer belt 11 to whichthe toner images developed on the photoconductor drum 41 are transferredtherefrom, a cleaning unit 45 for cleaning the photoconductor drum 41, acleaning unit 46 for cleaning the intermediate image transfer belt 11,and an image fixing unit 47 for fixing toner images on an image transfersheet P to which the toner images are secondarily transferred from theintermediate image transfer belt 11.

In the above-mentioned example, the case where two kinds of carbonparticles 13 and 14 are dispersed in the substrate 11 has beenexplained. However, three or more kinds of carbon particles withdifferent particle sizes may be dispersed in accordance with therequired electroconductivity,

Japanese Patent Application No. 10-270559 filed Sep. 8, 1998 andJapanese Patent Application No. 11-226277 filed Aug. 10, 1999, arehereby incorporated by reference.

What is claimed is:
 1. A method of producing an endless-belt-shaped filmwith a single layer structure comprising an insulating matrix resin andan electroconductive material dispersed in the form of needles in saidinsulating matrix resin with a longitudinal side of said needles of saidelectroconductive material being oriented in the direction normal to anexternal surface of said film, wherein a cross section of each of saidneedles of said electroconductive material in the direction normal tosaid external surface of said film is in the shape of a quadrilateralwith unequal diagonal lines, and wherein a longer diagonal line isoriented in the direction normal to said external surface of said filmand a shorter diagonal line is oriented in the direction normal to saidlonger diagonal line, comprising the steps of: dispersing saidelectroconductive material in a solution of said matrix resin in asolvent to form a dispersion of said electroconductive material in saidsolution of said matrix resin, and subjecting said dispersion tocentrifugal molding to orient a longitudinal side of said needles ofsaid electroconductive material in the direction normal to an externalsurface of said film, wherein a cross section of each of said needles ofsaid electroconductive material in the direction normal to said externalsurface of said film is in the shape of a quadrilateral with unequaldiagonal lines, and wherein a longer diagonal line is oriented in thedirection normal to said external surface of said film and a shorterdiagonal line is oriented in the direction normal to said longerdiagonal line, and removing said solvent therefrom.
 2. The method ofclaim 1, wherein said centrifugal molding is carried out in acentrifugal molding cylinder.
 3. The method of claim 1, wherein a stepof heating is carried out during said centrifugal molding.
 4. The methodof claim 1, wherein steps of drying and curing are carried out aftersaid centrifugal molding.
 5. The method as claimed in claim 1, whereinsaid cross section of each of said needles of said electroconductivematerial in the direction normal to said external surface of said filmis in the shape of a quadrilateral, wherein said quadrilateral has apair of equal adjacent sides directed to said external surface of saidfilm and a pair of equal adjacent sides directed to a back side of saidfilm.
 6. The method as claimed in claim 1, wherein said cross section ofeach of said needles of said electroconductive material in the directionnormal to said external surface of said film is in the shape of aquadrilateral, wherein said quadrilateral has a pair of equal adjacentsides of said quadrilateral directed to said external surface of saidfilm which are shorter than a pair of equal adjacent sides of saidquadrilateral directed to a back side of said film.
 7. The method asclaimed in claim 1, wherein said electroconductive material comprisesselenium.
 8. The method as claimed in claim 1, wherein saidelectroconductive material comprises carbon.
 9. The method as claimed inclaim 1, wherein said electroconductive material comprises a firstelectroconductive material and a second electroconductive material, andwherein the first and second electroconductive materials have differentresistivities.
 10. The method as claimed in claim 1, wherein saidelectroconductive material comprises a first electroconductive materialand a second electroconductive material, and wherein the first andsecond electroconductive materials have different particle sizes. 11.The method as claimed in claim 1, wherein the insulating matrix resincomprises at least one selected from the group consisting of polyimide,polyether sulfone, polycarbonate, polyester, polyarylate, polyphenylenesulfide, polyamide, polysulfone, polyparabanic acid, fluoroplastic,polyamide imide, polyether imide, thermosetting unsaturated polyester,and epoxy thermosetting resin.
 12. The method as claimed in claim 1,wherein the insulating matrix resin comprises polyimide.
 13. The methodas claimed in claim 1, wherein the insulating matrix resin comprisesfluoroplastic.
 14. The method as claimed in claim 1, wherein saidquadrilateral comprises a pair of adjacent sides, m, m′, n and n′, whichare directed to the external surface of said film and a pair of adjacentsides, n and n′, which are directed to a back side of said film, whichback side is opposite said external surface, and wherein m, m′, n and n′satisfy the following relationships: m=m′; and n=n′.
 15. The method asclaimed in claim 1, wherein said quadrilateral comprises a pair of equalsides, m, which are directed to the external surface of said film and apair of equal adjacent sides, n, which are directed to a back side ofsaid film, which back side is opposite said external surface, andwherein m and n satisfy the following relationship: m<n.